This invention pertains generally to the field of journal bearings and in particular to a tilting pad journal bearing in which the bearing contributes significantly to the overall damping of the supported rotating part without requiring substantial preloading of the pads against the journal.
Tilting pad bearings are commonly used in large high-speed rotating machinery such as gas and steam turbines and high-speed compressors. Although most applications of the tilting pad bearing are generally regarded as quite successful, it is recognized that, in some situations, the tilting pad bearing's capacity for vibration damping is less than desired. For example, tilting pad bearings used for a turbine rotor provide the only practical means by which vibrations in the rotor may be damped. It is apparent then that the damping characteristics of these bearings in that particular application are a very important consideration, and means for improving their damping capabilities have been eagerly sought by workers in the field.
One example of a tilting pad bearing presented as an improvement over more conventional designs - but not specifically directed to improved vibration damping - is that disclosed in U.S. Pat. No. 3,549,215 to Hollingsworth. In the bearing described in the '215 patent, each individual pad (typically, tilting pad bearings utilize 3-6 pads) is supported on a hydrodynamically generated oil film pressurized by high pressure oil bled from the lubricating film that exists between the journal and the pad. This arrangement is directed toward improving the support characteristic of the more heavily loaded pads by eliminating the mechanical pivot otherwise required for such pads and replacing this "hard" support with a more compliant one.
A high-speed bearing long known in the art as a predecessor to the tilting pad bearing, and designed principally for use in steam turbines and the like, is that disclosed in U.S. Pat. No. 2,584,770 to Wilcock. In the Wilcock bearing, a segment of the bearing is loaded by withdrawing oil from the lower, loaded segment for pressurizing the opposing, upper segment. The purpose of this particular bearing is to create a significant additional load on the journal to stabilize it against oil whip. It is notable that the Wilcock bearing is not a tilting pad type which, incidently, are inherently stable against oil whip.
Thus, while the foregoing discussion is believed to present the status of the prior art in the field of high speed bearings having particular utility in large rotary machines, it has been recognized that tilting pad bearings having improved damping characteristics would be very desirable for use in such machines. Accordingly, it is an objective of the present invention to provide a tilting pad bearing in which the normal, "hard" support for loaded pads is retained while the damping characteristics are improved without significantly altering any of the other operating characteristics of the bearing.